I gladly append my name to the long list of people who enjoyed your essay!
One remark.
It occurred to me that, if one takes a fully abstract version of the nested shell model (dropping any biosphere-oriented interpretation, and any labelling of the levels), and populates it with randomly moving identical balls, initially all packed at level zero, one still obtains an apparent progress, in the sense that, with time, more and more shells will be populated, even with perfectly symmetric gateways (this is the second law of thermodynamics in action.)
I think it would be important to stress the essential differences between this elementary, biology-free layered onion and the one that you (and Dennett) refer to - after realizing that *both* of them, in some sense, enable progress.
One crucial difference may be in the fact of letting balls differentiate and evolve: a more complex ball may be capable of exploiting energetic opportunities that are out of the reach of a less complex ball; and this new match between agent and environment creates a new opportunity for growth, a new life form, a new shell (if I understood correctly!)
Another attractive goal that your essay suggests (at least to me) is to come up with some very simple, stochastic or algorithmic *formal* model of interacting agents capable of capturing the agent-environment-energy relation mentioned above, then run a simulation to see whether some quantized orbitals of life forms emerge for free.
Final remark: I see interesting similarities, or compatibilities, between the nested shell model of progress and Teilhard de Chardin s views at the biosphere and the cosmos (as expressed in The Human Phenomenon, 1955). I refer in particular to the distinction he makes between the two energies - tangential and radial. He would imagine the first to be operational within a single layer, while the second would be responsible for the growth of complexity and the shift to the next upper layer.
However, following your reasoning, the radial energy may be regarded as completely illusional: vertical emergence would be a trivial side effect of horizontal activities (which does not make me particularly unhappy.)